A multilayer dielectric stack according to the present invention replaces prior art silicon dioxide as dielectric layer in integrated circuits.
When integrating a high-k dielectric with a gate electrode into a conventional CMOS gate stack, two major process/thermal stability issues need to be addressed and tackled. It is hereby important to                1. Reduce reaction(s) at the top interface between the high-k dielectric and the gate electrode such as polysilicon either during polysilicon deposition and/or during subsequent thermal processing and/or to        2. Minimize degradation of high-k material integrity due to crystallization, which leads to formation of structural defects such as oxygen vacancies and grain boundaries that interact with gate electrode deposition process leading to pathways of enhanced electrical leakage and/or of dopant/impurity diffusion.        
Based upon knowledge from conventional SiO2-based CMOS gate dielectric material (which is amorphous and 100% compatible with polysilicon gate), it is known that nitrogen is a key modifier introduced to improve the resistance against boron diffusion. Likewise, in high-k dielectric, enhanced stability against dopant (boron) diffusion from the polysilicon gate can be achieved through the use of a metal-oxynitride (MOxNy) or nitridation of metal oxide (U.S. Pat. No. 6,251,761). U.S. Pat. No. 6,251,761 describes a gate stack including a gate dielectric with reduced effective electrical thickness. A high-k dielectric is deposited over the silicon substrate. In a next step, remote plasma nitridation of the high-k dielectric layer is performed to convert the upper surface of the high-k dielectric into a nitride layer. A conductive layer is formed over the nitride layer to complete the CMOS gate stack.
Published U.S. patent application No. 2002/0130340 A1 discloses a multilayer dielectric stack, which has alternating layers of high-k material and an interposing material. The interposing material can be Al2O3, AlN, SiN, Si3N4 and SiO2. The interposing material can be deposited by atomic layer deposition, sputtering or evaporation, resulting in the deposition of the interposing layer on the dielectric layer. Each layer is preferably less tan 50 Angstroms thick. This structure inhibits and prevents the crystallization of the high k-material.